There is provided a transition element for connecting a tower to a jacket. More precisely there is provided a transition element between a jacket having four legs and a tower where the lower portion of the tower is fixed to a torsion plate that is connected to the upper portion of the legs, and a brace extending from each leg where each brace includes a strut that is fixed to the upper portion of the leg and a plate that is fixed to the tower at a contact portion above the torsion plate.
The device is below illustrated by means of a supporting tower structure of a windmill, a structure of this kind adequately showing the challenges addressed by the invention. The invention is not limited to a tower for windmills, but can be used in a number of structures, in which similar force patterns exist.
To avoid collision with blades of the windmill, the upper portion of a windmill tower must be formed as a slim structure. From land-based windmills it is known that this slim structure, which is often in the form of a pipe, is connected to a base in the ground.
When windmills are placed off shore and mare often in relatively deep water, it is not practical to use one pipe extending from the base of the windmill and up to the nacelle of the windmill.
Therefore, the supporting tower structure of known windmills that are placed off shore, often has a tubular upper tower portion and a lower portion, in which the lower portion may be in the form of a jacket.
The transition element between the upper slim tower portion and the lower jacket is often formed by a relatively heavy and complicated structure. The reason is, among other things, that construction principles that are known from sea-based oil recovery equipment are applied. Such equipment is dimensioned for considerable wave forces and for handling heavy equipment, and for allowing personnel to be present on the structure at all times.
In windmill installations off shore, the design of the structure is normally determined by the wind forces.
Known transition elements have to be adjusted in such a way that the natural period of the tower will be short enough for the windmill concerned. This contributes to a further increase in the weight of the tower.